UNITED NATIONS — The U.N. special envoy for Yemen said Thursday the latest offensive by Yemen's Houthi rebels in the oil-rich central province of Marib threatens peace prospects, but he called the new U.S. administration's backing for international efforts to end the six-year war a new opportunity to reopen negotiations.

Martin Griffiths told the U.N. Security Council that he sees "common ground for agreement" between the Houthis and the internationally recognized government to end the conflict. "But there is nothing anybody can do to force the warring parties to peace, unless they choose to put down the guns and talk to each other," he said.

As the offensive is underway in Marib, where about 1 million Yemenis have fled since 2015 to get away from fighting elsewhere, "Yemen is speeding towards the worst famine the world has seen in decades," U.N. humanitarian chief Mark Lowcock warned.

He told the council some 400,000 children under the age of 5 are severely malnourished. "Across Yemen, more than 16 million people are going hungry — including 5 million who are just one step away from famine," he said.

The U.N. is seeking $4 billion for Yemen's humanitarian crisis this year and Secretary-General Antonio Guterres is convening a high-level pledging event March 1 co-hosted by Sweden and Switzerland, Lowcock said.

Griffiths was unusually grim on Thursday. "I want to emphasize what is at stake," he said. "The military situation in the country is extremely tense" — more so than at any time during his three years as Guterres' special representative to Yemen.

"Over the past month, the conflict in Yemen has taken a sharp escalatory turn" with the Houthis' offensive in Marib, he said.

"I have condemned this many times ... and I repeat my call now: The attack on Marib must stop," Griffiths said. "It puts millions of civilians ... at risk, especially with the fighting threatening to reach the camps for internally displaced persons."

He said civilians are bearing the brunt of hostilities and pointed to "shocking violations" of international humanitarian law.

The media office for the Iran-backed Houthis responded, saying they "consider the international position on the developments in Marib a cover-up for the continuation of aggression" by the Saudi-led coalition fighting the rebels in Yemen."

"Our people will not stay hands tied," the statement said. "They have the right to carry on their battle for liberation."

Yemen's war started in 2014 when the Houthis seized the capital, Sanaa, and much of the country's north. The Saudi-led, U.S.-backed coalition intervened months later to dislodge the rebels and restore the internationally recognized government. The conflict has killed some 130,000 people and spawned the world's worst humanitarian disaster.

Along with the Marib fighting, Griffiths said there have been "worrying spikes of violence and hostilities" in the provinces of Hodeida, where a key port is located, and the government-held Taiz.

Cross-border attacks into Saudi Arabia, blamed on the Houthis and their supporters, "damage prospects for peace and regional stability," Griffiths told council members.

He welcomed President Joe Biden's renewed focus on ending Yemen's war and the appointment of a new U.S. special envoy, Timothy Lenderking.

"This offers us new opportunity to reopen space for a negotiated solution," he said. "To seize this chance to revitalize the political process, the parties should immediately agree to a nationwide cease-fire."

Griffiths recently met with Yemeni government leaders, the Saudis, the Iranians and Lenderking, and has been in contact with the Houthis. He said he will continue engaging the parties "on the path forward, and I hope they will demonstrate the will to make progress."

On a positive note, Griffiths said negotiations between the two sides on a second release of prisoners and detainees have been taking place in Amman, Jordan for the past three weeks.

"We're not quite there yet," he said, urging the parties to successfully conclude negotiations. "And I call for the immediate and unconditional release of all sick, wounded, elderly and children detainees as well as all arbitrarily detained civilians including women and journalists."

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Associated Press writer Noha ElHennawy in Cairo contributed to this report.